[PATCH vhost v9 3/6] virtio: find_vqs: pass struct instead of multi parameters

[Michael S. Tsirkin]

On Thu, Jun 20, 2024 at 06:43:30PM +0800, Xuan Zhuo wrote:
> On Thu, 20 Jun 2024 06:15:08 -0400, "Michael S. Tsirkin" <mst at redhat.com> wrote:
> > On Thu, Jun 20, 2024 at 05:20:49PM +0800, Xuan Zhuo wrote:
> > > On Thu, 20 Jun 2024 05:14:24 -0400, "Michael S. Tsirkin" <mst at redhat.com> wrote:
> > > > On Thu, Jun 20, 2024 at 05:00:49PM +0800, Xuan Zhuo wrote:
> > > > > > > @@ -226,21 +248,37 @@ struct virtqueue *virtio_find_single_vq(struct virtio_device *vdev,
> > > > > > >
> > > > > > >  static inline
> > > > > > >  int virtio_find_vqs(struct virtio_device *vdev, unsigned nvqs,
> > > > > > > -			struct virtqueue *vqs[], vq_callback_t *callbacks[],
> > > > > > > -			const char * const names[],
> > > > > > > -			struct irq_affinity *desc)
> > > > > > > +		    struct virtqueue *vqs[], vq_callback_t *callbacks[],
> > > > > > > +		    const char * const names[],
> > > > > > > +		    struct irq_affinity *desc)
> > > > > > >  {
> > > > > > > -	return vdev->config->find_vqs(vdev, nvqs, vqs, callbacks, names, NULL, desc);
> > > > > > > +	struct virtio_vq_config cfg = {};
> > > > > > > +
> > > > > > > +	cfg.nvqs = nvqs;
> > > > > > > +	cfg.vqs = vqs;
> > > > > > > +	cfg.callbacks = callbacks;
> > > > > > > +	cfg.names = (const char **)names;
> > > > > >
> > > > > >
> > > > > > Casting const away? Not safe.
> > > > >
> > > > >
> > > > >
> > > > > Because the vp_modern_create_avq() use the "const char *names[]",
> > > > > and the virtio_uml.c changes the name in the subsequent commit, so
> > > > > change the "names" inside the virtio_vq_config from "const char *const
> > > > > *names" to "const char **names".
> > > >
> > > > I'm not sure I understand which commit you mean,
> > > > and this kind of change needs to be documented, but it does not matter.
> > > > Don't cast away const.
> > >
> > >
> > > Do you mean change the virtio_find_vqs(), from
> > > const char * const names[] to const char *names[].
> > >
> > > And update the caller?
> > >
> > > If we do not cast the const, we need to update all the caller to remove the
> > > const.
> > >
> > > Right?
> > >
> > > Thanks.
> >
> >
> > Just do not split the patchset at a boundary that makes you do that.
> > If you are passing in an array from a const section then it
> > has to be const and attempts to change it are a bad idea.
> 
> Without this patch set:
> 
> static struct virtqueue *vu_setup_vq(struct virtio_device *vdev,
> 				     unsigned index, vq_callback_t *callback,
> 				     const char *name, bool ctx)
> {
> 	struct virtio_uml_device *vu_dev = to_virtio_uml_device(vdev);
> 	struct platform_device *pdev = vu_dev->pdev;
> 	struct virtio_uml_vq_info *info;
> 	struct virtqueue *vq;
> 	int num = MAX_SUPPORTED_QUEUE_SIZE;
> 	int rc;
> 
> 	info = kzalloc(sizeof(*info), GFP_KERNEL);
> 	if (!info) {
> 		rc = -ENOMEM;
> 		goto error_kzalloc;
> 	}
> ->	snprintf(info->name, sizeof(info->name), "%s.%d-%s", pdev->name,
> 		 pdev->id, name);
> 
> 	vq = vring_create_virtqueue(index, num, PAGE_SIZE, vdev, true, true,
> 				    ctx, vu_notify, callback, info->name);
> 
> 
> The name is changed by vu_setup_vq().
> If we want to pass names to
> virtio ring, the names must not be  "const char * const"
> 
> And the admin queue of pci do the same thing.
> 
> And I think you are right, we should not cast the const.
> So we have to remove the "const" from the source.
> And I checked the source code, if we remove the "const", I think
> that makes sense.
> 
> Thanks.

/facepalm

This is a different const.


There should be no need to drop the annotation, core
does not change these things and using const helps make
sure that is the case.



> 
> 
> >
> >
> > > >
> > > > --
> > > > MST
> > > >
> >